Grinding and/or dispersing agent containing polymers and/or copolymers partially neutralized by magnesium for aqueous suspensions of mineral substances, to be used in pigmentary applications

ABSTRACT

A grinding agent containing polymers and/or copolymers partially neutralized by magnesium, for aqueous suspensions of mineral substances to be used in pigmentary applications is disclosed. 
     A grinding agent composed of polymers and/or copolymers having a specific viscosity of between 0.3 and 0.8 and obtained by treatment, using one or several solvents, of a polymer and/or copolymer resulting from polymerization and/or copolymerization of acrylic and/or vinyl monomers comprising acid groups whose active acid sites are partially or completely neutralized to a degree of neutralization of 40 to 60%, including terminals, of the active acid sites using a neutralizing agent containing magnesium ions, and a degree of neutralization reaching 60% using a neutralizing agent containing sodium ions is disclosed. 
     A procedure for grinding in an aqueous suspension using said agent, as well as aqueous suspensions produced using said procedure and their applications to the fields of weight fillers, paper coating, paints, and plastics is also disclosed.

This is a continuation, of application Ser. No. 07/974,271 filed on Nov.10, 1992 U.S. Pat. No. 5,432,238.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a grinding and/or dispersing agent, amethod of grinding and/or dispersing with such an agent, and adispersion containing said agent.

2. Discussion of the Background

For a long time, conventional practice has included the use of mineralsubstances such as calcium carbonates and titanium dioxide for thepreparation of industrial products intended for paints, paper coating,fillers for rubbers and synthetic resins, and others.

However, since these mineral substances do not have a natural lamellaror laminated structure facilitating their splitting, as is the case forcertain substances such as aluminum silicates, generally known askaolin, the specialist must, in order to use them in the pigment field,transform them by grinding in a very fine aqueous suspension whoseconstituent grains are as small as possible, i.e., of less than severalmicrons.

Through the publications that have appeared in this area, thespecialized literature reveals the importance and complexity of grindingin an aqueous medium of mineral substances, so as to obtain a quality ofpartial fineness allowing pigmentary applications. Thus, it is wellknown, in the special case of paper coating, that the coating materialcomposed of mineral pigments such as kaolins, calcium carbonates, andtitanium dioxides are placed in suspension in water, also containsbinding agents and dispersants, as well as other additives such asthickening agents and coloring agents.

It is desirable to ensure that a suspension of this kind possesses alow, stable viscosity for the length of time it is stored, so as tofacilitate handling and use, and, similarly, has as high a mineralsubstance content as possible, in order to reduce the quantity of waterhandled. A suspension, ideally uniting all of these basic properties,would allow the artisan to solve the well-known problems of grinding,storage, transport from the production site to the application site,and, finally, of transfer by pump during use.

Thus, to date those of skill in the art have been familiar with the useof grinding and/or aqueous dispersing agents for mineral particlesformed by acrylic polymers and/or copolymers which are totally orpartially neutralized by various neutralization agents (Patents Nos.FR2539137, FR2531444, FR2603042).

The artisan is also familiar with the use of a grinding and/ordispersing agent formed from the fraction of alkali acrylic polymersand/or copolymers which is totally neutralized by a single cation, andwhose specific viscosity is between 0.3 and 0.8 (Patent No. FR2488814),or which is partially neutralized by a single cation such as sodium,potassium, lithium, or ammonium (EP0127388; EP0185458).

These various types of grinding and/or dispersing agents, according toprior art and other treatment or grinding procedures known to theartisan (EP0278880; FR2642415), make it possible to obtain aqueoussuspensions of fine mineral particles, which have been thought, untilnow, to be stable over time, but for which the user experiences somehandling problems, since they are often stored for several days or weekswithout stirring, and thus must deal with an increase in viscosity ofthe unstirred suspension.

In fact, in the prior art, the viscosity of the aqueous suspension ofmineral particles was generally measured using a Brookfield RVT-typeviscosimeter at a temperature of 20° C. and a rate of rotation of theNo. 3 moving component of 100 revolutions/minute:

(a) at the grinding apparatus outlet;

(b) after eight days of rest in the bottle, and after preliminarystirring of the resting suspension.

These two measurement parameters did not make it possible to bring tolight "the increase of viscosity" of the aqueous mineral suspension,which is harmful to the user, who must handle concentrated suspensionswhich have been stored for several days without being stirred, and whichis characterized by a Brookfield viscosity before stirring of more than2000 mPa·s, obtained by measuring at 10 revolutions/minute.

Faced with these problems, the present inventors have found that aqueousmineral suspensions obtained according to prior art had high Brookfieldviscosities if the measurement was made after extended storage withoutstirring the suspension preliminarily, as is generally the case the userof these suspensions is faced with.

Based on this finding, the present inventors have developed a grindingand/or dispersing agent and a grinding procedure in the presence of thisagent, which, surprisingly, makes it possible to produce aqueous mineralsuspensions meeting the aforementioned quality criteria.

Surprisingly, the objectives of the invention are achieved when between40-60% of the active acid sites of an acrylic polymer, includingterminals, are neutralized by a neutralization agent containingmagnesium ions, while less than or equal to 60% of the active acid sitesof the polymer are neutralized by a neutralizing agent containing sodiumions.

SUMMARY OF THE INVENTION

Thus, one object of the invention is to provide an improved grindingand/or dispersing agent.

Another object of the invention is to supply an improved grinding and/ordispersing agent in an aqueous suspension of coarse mineral substances,whose viscosity remains substantially the same over time.

A further object of the invention is to supply a grinding procedure inan aqueous suspension of mineral particles.

The object of the present invention is provided for by an improvedgrinding and/or dispersing agent in an aqueous suspension of coarsemineral substances, making it possible to obtain an aqueous suspensionof fine mineral particles for pigmentary applications, such as anaqueous calcium carbonate suspension whose dry matter content is atleast 70% by weight, in which the size of at least 90% of the essentialparticles is less than two μm after grinding, while the size of 60% ofthese particles is less than one μm.

The invention also provides for a grinding procedure in an aqueousmedium of coarse mineral substances in the presence of saidwater-soluble grinding agent. This procedure gives aqueous suspensionsof mineral substances whose viscosity remains stable over time, even ifstirring of the suspensions should not be maintained, thereby ensuringsignificant ease of handling and use. This procedure is particularlywell suited to grinding of an aqueous calcium carbonate suspension,whose dry matter content is at least 70% by weight, in which the size ofat least 90% of the essential particles after grinding is less than twoμm, while the size of at least 60% of these particles is less than oneμm. Moreover, this invention concerns aqueous suspensions of mineralparticles obtained by means of this procedure, and their applications.

These suspensions according to the invention, which are intended forpigmentary applications in which the size of at least 90% of theparticles is less than two μm, and in which at least 60% of them measureless than one μm, have a low, stable viscosity over time, even when notstirred, i.e., a viscosity after resting for eight day of approximatelythe same value as that obtained at the outlet of the grinder followingthe grinding operation.

A final purpose of the invention is the use of these fine aqueousmineral suspensions in the fields of weight fillers, paper coating, andpaints, and, after potential removal of the aqueous phase, in the fieldof polymer materials.

These objectives are achieved through the use, in the grinding processaccording to the invention, of the invention agent, which is produced bypolymerization and/or copolymerization of acrylic and/or vinyl monomers,followed by fractionation by means of one or several polar solvents, inaccordance with procedures known to the artisan.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This agent according to the invention is characterized by the fact thatit is formed from the fraction of said acid polymers and/or copolymers,whose active acid sites are partially or completely neutralized,undergoing a degree of neutralization of the active acid sites ofbetween 40 and 60%, including terminals, by means of a neutralizationagent incorporating magnesium ions, and a degree of neutralization ofthe active acid sites capable of reaching 60% using a neutralizationagent incorporating sodium ions.

These acrylic polymers and/or copolymers are obtained from conventionalradical polymerization procedures in the presence of polymerizationregulators, such as hydroxylamine-based organic compounds and in thepresence of polymerization catalysts such as peroxides and persalts,e.g., oxygenated water persulfate, sodium hypophosphite, hypophosphorousacid. The following monomers and/or co-monomers may be used: acrylicacid, methacrylic acid, itaconic acid, crotonic acid, fumaric acid,maleic anhydride acid, or, alternatively, isocrotonic acid, aconiticacid (cis or trans), mesaconic acid, sinapinic acid, undecylenic acid,angelic acid, canellic acid, or hydroxyacrylic acid, existing either asfree acids or partially neutralized salts, acrolein, acrylamide,acrylonitrile, the esters of acrylic and methacrylic acids, and, inparticular, dimethylaminoethyl methacrylate, vinylpyrrolidone,vinylcaprolactam, ethylene, propylene, isobutylene, diisobutylene, vinylacetate, styrene, α-methyl styrene, and methyl vinyl ketone.

As a suitable polymerization medium may be water, methanol, ethanol,propanol, isopropanol, the butanols, or mixtures of these, or,alternatively, dimethylformamide, dimethyl sulfoxide, tetrahydrofuran,acetone, methyl ethyl ketone, ethyl acetate, butyl acetate, hexane,heptane, benzene, toluene, xylene, mercaptoethanol,tertiododecylmercapton, thiogylycolic acid and its esters,n-dodecylmercaptan, acetic acid, tartaric acid, lactic acid, citricacid, gluconic acid, glucoheptonic acid, and 2-mercaptopropionic acids,thiodiethanol, halogenated solvents such as carbon tetrachloride,chloroform, methylene chloride, and monopropylene glycol ordiethyleneglycol ethers or mixtures of these latter.

The solution of the acid polymerization product thus obtained ispotentially neutralized, partially or completely, to a degree of 40 to60% using a neutralizing agent containing magnesium ions, and in aproportion of less than, or equal to, 60% using a neutralizing agentcontaining sodium ions.

The solution of the polymerization product, either acidic or neutralizedin this manner, is then treated, using static or dynamic proceduresknown in the art, by one or several polar solvents belonging, inparticular, to the group of methanol, ethanol, propanol, isopropanol,the butanols, acetone, and tetrahydrofuran, thereby producing atwo-phase separation: --the least dense phase, containing the majorfraction of the polar solvent and the acrylic polymer and/or copolymerfraction having low molecular weight, and--the densest aqueous phasecontaining the acrylic polymer and/or copolymer fraction having thehighest molecular weight.

The desirable fraction composed of the acrylic polymers and/orcopolymers according to the invention, whose specific viscosity isbetween 0.3 and 0.8, is collected.

This specific viscosity of the acrylic polymers and/or copolymers,symbolized by the letter "η" is determined in the following way.

A sample of the solution of the polymer and/or copolymer fractioncollected is taken so as to obtain a solution corresponding to 2.5g ofdry polymer and to 50 ml of a 60 g/1 solution of sodium chloride. Next,a capillary viscosimeter with a Baume constant of 0.000105 is placed ina bath thermostat controlled to 25° C. which is used to measure the flowtime of a given volume of the aforementioned solution containing theacrylic polymer and/or copolymer, and the flow time of the same volumeof aqueous solution of sodium chloride from which said polymer and/orcopolymer is absent. The viscosity "η" can then be calculated by meansof the following equation. ##EQU1##

The capillary tube is generally chosen so that the flow time of the NaClsolution not containing the polymer and/or copolymer, will beapproximately 90 to 100 seconds, thus giving very accurate specificviscosity measurements.

The temperature at which selection treatment of the acidic or alkaliacrylic polymer and/or copolymer is effected is not crucial in itself,since it influences the coefficient of partition alone. It rangesbetween 10 and 80° C., and preferably between 20° C. and 60° C.

On the other hand, the ratio of the quantities of diluted water and ofpolar solvents is of prime importance, since it affects separationdirectly. Similarly, when separation is effected continuously using acentrifugal extractor, i.e., using a dynamic process, the ratios of theextracted fractions depends on centrifuging conditions.

It is also possible, and desirable, in certain cases, to further refinethe selection of the fraction of the acrylic polymers and/or copolymers,by retreating the densest aqueous phase previously collected, using anew quantity of polar solvent which may be different from that, or thosepreviously used, or which may be a mixture of polar solvents.

The liquid phase after treatment can be distilled to remove the solventor solvents used for treatment.

The aqueous phase containing the fraction of acrylic polymers and/orcopolymers whose specific viscosity ranges between 0.3 and 0.8 andcorresponding to a mean molar mass by weight (Mw) of between 1,000 and10,000 g/mol, can be employed in this form as a grinding agent formineral substances to be ground; however, it can also be treated usingany conventional means to remove its aqueous phase and isolate theacrylic polymer and/or copolymer in the form of a fine powder, which maybe used in that other form as a grinding agent.

In practice, the operation for grinding the mineral substance to befined consists of grinding the mineral material with a grindingsubstance into very fine particles in an aqueous suspension containingthe grinding agent. An aqueous suspension of the mineral substance to beground is formed, in which the initial size of the grains is at most 50μm, in a quantity such that the concentration in dry matter of thissuspension is at least 70% by weight.

The grinding substance, whose granulometry is advantageously between0.20 millimeter and 4 millimeters, is added to the suspension of themineral substance to be ground. The grinding substance generally existsas particles of materials as varied as silicon oxide, aluminum oxide,zirconium oxide or mixtures of these latter, as well as very hardsynthetic resins, steels, etc. An example of the composition of grindingsubstances of this kind is given in Patent No. FR 2203681, whichdescribes grinding elements composed of from 30 to 70% by weight ofzirconium oxide, 0.1 to 5% of aluminum oxide, and from 5 to 20% ofsilicon oxide. The grinding substance is preferably added to thesuspension in a quantity such that the ratio by weight of the grindingmaterial to the mineral substance to be ground is at least 2/1, thisratio preferably falling between the limits of 3/1 and 5/1.

The mixture of the suspension and of the grinding substance is thensubjected to mechanical stirring, such as that generated in aconventional grinder incorporating micro-components.

The grinding and/or dispersing agent according to the invention is alsoadded to the mixture formed by the aqueous suspension of mineralsubstances and by the grinding substance, in a proportion of 0.2 to 2%by weight of the dried fraction of said polymers, in relation to the dryweight of the mineral substance to be fined.

The time required to achieve a high level of fineness of the mineralsubstance after grinding varies, depending on the nature and quantity ofthe mineral substances to be ground and on the stirring method used andthe temperature of the medium during the grinding operation.

The mineral substances to be fined may come from very diverse sources,such as calcium carbonate and the dolomites, calcium sulfate, kaolins,and titanium dioxide, i.e., all mineral substances which must be groundto be usable in applications as various as paper coating, pigmenting ofpaints and coatings, fillers for rubbers and synthetic resins, impartinga dull finish to synthetic textiles, etc.

Thus, the application of the grinding and/or dispersing agent accordingto the invention allows the transformation, by grinding in an aqueoussuspension containing a high concentration of dry matter, of coarsemineral substances into very fine particles, in which the size of atleast 90% of these particles is always smaller than two μm, and in whichthe size of at least 60% is less than one μm; and this application makesit possible to obtain a suspension of very fine mineral substances whoseviscosity is low and stable over time.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLE 1

The example involves the preparation of a suspension of coarse calciumcarbonate subjected to grinding to fine it into a microparticulatesuspension. To this end, a suspension of coarse calcium carbonate wasprepared from a natural calcium carbonate having an average diameter of50 micrometers, by using:

for Test No. 1,illustrating prior art, the densest fraction of a sodiumpolyacrylate obtained from isopropanol fractionation of a sodiumpolyacrylate whose specific viscosity of 0.525 and obtained by radicalpolymerization and total neutralization (100% of the active acid sites)using sodium hydroxide;

for Test no. 2, also illustrating prior art, the densest fraction of amixed sodium and calcium polyacrylate obtained by isopropanolfractionation of a mixed sodium and calcium polyacrylate having aspecific viscosity of 0.525, obtained by radical polymerization andtotal neutralization, using sodium hydroxide and lime in a ratiocorresponding to neutralization of the active acid sites, in theproportion of 70% sodium to 30% calcium;

for Test No. 3, still illustrating prior art, the densest fraction of asodium polyacrylate obtained by isopropanol fractionation of a sodiumpolyacrylate whose specific viscosity is 0.525 and which is obtained byradical polymerization and partial neutralization (66% of the activeacid sites) using sodium hydroxide;

for Test No. 4, which illustrates the invention, the densest fraction ofa mixed sodium and magnesium polyacrylate obtained by isopropanolfractionation of a mixed sodium and magnesium polyacrylate whosespecific viscosity is 0.525 and which is obtained by totalneutralization (100% of the active acid sites) using sodium hydroxideand magnesium hydroxide, in a ratio corresponding to neutralization ofthe active acid sites in a proportion of 50% by the sodium to 50% by themagnesium.

For each test, an aqueous suspension of calcium carbonate taken from theOrgan (France) deposit and having a granulometry of less than 10 micronswas prepared.

This suspension had a dry matter concentration of 76% by weight of thetotal weight.

The grinding agent was added to this suspension in the quantitiesindicated in the following table, expressed as percent of dry weight ofthe Torac weight of the dry calcium carbonate to be ground.

The suspension circulated in a Dyno-Mill grinder incorporating astationary cylinder and a rotating pulse generator, in which thegrinding substance was corundum balls having a diameter of between 0.6millimeter and 1.0 millimeter.

The total volume taken up by the grinding substance was 1,150 cubiccentimeters, and its weight, 2,900 g.

The grinding chamber had a volume of 1,400 cubic centimeters.

The circumferential speed of the grinder was 10 meters/second.

The calcium carbonate suspension was recycled at the rate of 18liters/hour.

The outlet of the Dyno-Mill grinder was equipped with a 200-micron meshseparator allowing separation of the ground suspension and the grindingsubstance.

The temperature during each grinding test was kept at approximately 60°C.

Following the grinding operation (To), a sample of the pigmentarysuspension, in which the size of 80% of the particles was less than onemicron, was collected in a bottle, and the viscosity was measured usinga Brookfield RVT viscosimeter at a temperature of 20° C. and a speed ofrotation of 10 revolutions/minute with the suitable moving component.

After the suspension had rested in the bottle for eight days, theviscosity was measured by inserting into the unstirred bottle thesuitable moving component of the Brookfield RVT viscosimeter, at atemperature of 20° C., and a speed of rotation of 10 revolutions/minute(AVAG viscosity: before stirring).

All experimental results are recapitulated in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    GRINDING AGENT USED         Brookfield Viscosity of                           NEUTRALIZATION              the suspension (at 20° C.                  Percentage                  in mPA · s)                                   of acid    Neutral-                                                                           Consumption AVAG after                                        groups neu-                                                                              izing                                                                              of agent in                                                                          To 10                                                                              eight days                                   Test No.                                                                           tralized   cation                                                                             %/dry/dry                                                                            T/mn 10 T/mn                                      __________________________________________________________________________    1*   100        Na   0.86   1,400                                                                              8,290                                        2*   70/30      Na/Ca                                                                              1.05   1,250                                                                              2,390                                        3*    66        Na   1.25   1,500                                                                              6,750                                        4**  50/50      Na/Mg                                                                              0.98   1,040                                                                              1,040                                        __________________________________________________________________________     *Prior Art                                                                    **Invention                                                                   AVAG: Measurement of the viscosity prior to stirring of the suspension   

A reading of Table 1 reveals that only the calcium carbonate groundusing the invention agent (Test No. 4) has a Brookfield viscosity, asmeasured at 10 revolutions/minute after eight days at rest (withoutpreliminary stirring), which is appreciably less than 2,000 mPa·s, whileall of the tests of the examples carried out using an agent according toprior art give viscosities of more than 2,000 mPa·s.

Thus, a reading of Table 1 discloses the increased viscosity insuspensions of ground calcium carbonate using agents according to priorart, while the grinding and/or dispersing agent according to theinvention makes it possible to obtain stability of the viscosity of thecalcium carbonate suspensions even prior to stirring,

EXAMPLE 2

A second group of tests (Tests Nos. 5-12) involves preparation andgrinding of an aqueous suspension of the same calcium carbonate at thesame dry matter concentration in the presence of the densest fractionobtained by isopropanol fractionation of the same polyacrylic acidhaving a specific viscosity of 0.525 and neutralized using variableproportions of magnesium.

These tests were conducted based on the same experimental criteria asthose in Example 1.

All experimental results appear in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    GRINDING AGENT USED         Brookfield Viscosity of                           NEUTRALIZATION              the suspension (at 20° C.                  Percentage                  in mPA · s)                                   of acid    Neutral-                                                                           Consumption AVAG after                                        groups neu-                                                                              izing                                                                              of agent in                                                                          To 10                                                                              eight days                                   Test No.                                                                           tralized   cation                                                                             %/dry/dry                                                                            T/mn 10 T/mn                                      __________________________________________________________________________     5   70/30      Na/Mg                                                                              1.05   1,330                                                                              3,590                                         6** 60/40      Na/Mg                                                                              1.03   1,200                                                                              1,900                                         7** 55/45      Ma/Mg                                                                              0.9    1,210                                                                              1,510                                         8** 50/50      Na/Mg                                                                              0.98   1,040                                                                              1,040                                         9** 45/55      Na/Mg                                                                              1.02   1,250                                                                              1,190                                        10** 40/60      Na/Mg                                                                              1.09   1,390                                                                              1,635                                        11   30/70      Na/Mg                                                                              1.16   1,160                                                                              1,730                                        12   50/45      Na/Mg                                                                              1.08   1,420                                                                              1,900                                        __________________________________________________________________________     **Invention                                                                   AVAG: Measurement of the viscosity prior to stirring of the suspension   

A reading of Table 2 reveals the extraordinary rheological resultsyielded by the invention, and the fact that the use of the agentaccording to the invention makes it possible to cross the threshold ofprofitability during practical tests.

Indeed, two criteria are considered by the user, the economic and therheological aspects. An excess quantity of the grinding agent entails anexcess cost of production of the fine calcium carbonate suspensions.

Thus, consumption of the grinding and/or dispersing agent in thegrinding operation based on the invention must not exceed approximately1.1% of the dry dispersing agent in relation to the dry weight of thecalcium carbonate, so as to give a suspension in which the size of 80%of the particles is less than one micron and the concentration of drymatter is 76%. Tests Nos. 6 and 10 thus establish the limits of theinvention.

As regards rheology, the user, who must handle concentrated suspensionsstored for several days without being stirred, wishes to havesuspensions whose viscosity is virtually identical to that of thesuspensions leaving the grinder, this viscosity always being less than2,000 mpa·s. This is the extraordinary result obtained for the firsttime using the agent based on the invention (Tests Nos. 6 to 10 and 12,and, in particular, Tests 7 to 9).

EXAMPLE 3

This example (Tests Nos. 13 and 14) involves the preparation andgrinding of an aqueous suspension of the same calcium carbonate at thesame dry matter concentration and in the presence of the densestfraction obtained by isopropanol fractionation of the same polyacrylicacid having a specific viscosity of 0.525 and neutralized usingdifferent neutralizing agents containing polyfunctional ions other thanmagnesium, e.g., calcium.

Thus, in Test No. 13 grinding is carried out using the same fraction ofpolyacrylic acid as that mentioned above, which is neutralized withsodium hydroxide and lime in a ratio corresponding to neutralization ofthe active acid sites, in a proportion of 70% by the sodium and 30% bythe calcium. Test No. 14 involves the same fraction of polyacrylic acidneutralized by sodium hydroxide and lime in a ratio corresponding toneutralization of 55% of the active acid sites by means of the sodium,and 45% by means of the calcium. 20 These tests were conducted based onthe same experimental criteria as those applied in Example 1.

All results are recorded in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    GRINDING AGENT USED         Brookfield Viscosity of                           NEUTRALIZATION              the suspension (at 20° C.                  Percentage                  in mPA · s)                                   of acid    Neutral-                                                                           Consumption AVAG after                                        groups neu-                                                                              izing                                                                              of agent in                                                                          To 10                                                                              eight days                                   Test No.                                                                           tralized   cation                                                                             %/dry/dry                                                                            T/mn 10 T/mn                                      __________________________________________________________________________    13   70/30      Na/Ca                                                                              1.05   1,250                                                                              2,390                                        14   55/45      Na/Ca                                                                              1.3    1,500                                                                              3,300                                         7** 55/45      Na/Mg                                                                              0.9    1,210                                                                              1,510                                        __________________________________________________________________________     **Invention                                                                   AVAG: Measurement of the viscosity prior to stirring of the suspension   

A reading of Table 3 reveals that use of another neutralizingpolyfunctional cation such as calcium, even in different proportions,does not yield a fine calcium carbonate suspension having a Brookfieldviscosity, as measured at 10 revolutions/minute after eight days at restand without preliminary stirring, of less than 2,000 mPa·s (Tests Nos.13 and 14).

EXAMPLE 4

This example (Tests Nos. 15-17) involves the preparation and grinding ofan aqueous suspension of the same calcium carbonate at the same drymatter concentration and in the presence of the densest fractionobtained by isopropanol fractionation of the same polyacrylic acidhaving a specific viscosity of 0.525 and neutralized at a rate of 50% bythe magnesium ion and 50% by using different monovalent neutralizingagents.

These different monovalent neutralizing agents are, in Test No. 15,ammonium, lithium in Test No. 16, and potassium in Test No. 17.

These tests were conducted using the same experimental criteria as thoseused in Example 1.

All results are recorded in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    GRINDING AGENT USED         Brookfield Viscosity of                           NEUTRALIZATION              the suspension (at 20° C.                  Percentage                  in mPA · s)                                   of acid    Neutral-                                                                           Consumption AVAG after                                        groups neu-                                                                              izing                                                                              of agent in                                                                          To 10                                                                              eight days                                   Test No.                                                                           tralized   cation                                                                             %/dry/dry                                                                            T/mn 10 T/mn                                      __________________________________________________________________________    15   50/50      NH.sub.4 /Mg                                                                       It is not possible to obtain a                                                granulometry equivalent to                                                    80% < 1 μm                                            16   50/50      Li/Mg                                                                              0.89   1,620                                                                              3,800                                        17   50/50      K/Mg 1.11   1,800                                                                              2,700                                        __________________________________________________________________________     AVAG: Measurement of the viscosity prior to stirring of the suspension.  

A reading of Table 4 reveals that the sodium ion cannot advantageouslybe replaced by another monovalent ion.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by letters patent ofthe U.S. is:
 1. An aqueous suspension of fine mineral substances refinedby a process for grinding an aqueous suspension of coarse mineralsubstances intended for pigmentary applications, comprising:(i)preparing an aqueous suspension of a coarse mineral substance; (ii)adding a grinding agent comprising: a polymer selected from the groupconsisting of acrylic polymers, vinyl polymers, acrylic and vinylcopolymers, and a mixture thereof: wherein said polymer contains acidgroups which are partially or completely neutralized and is obtained bytreatment, using one or several polar solvents according to static ordynamic procedures, and by the selection of the fraction of having thedesired molecular weight, wherein the active acid sites of said polymerare completely or partially neutralized such that the percentage of saidactive acid sites neutralized by magnesium ions ranges between 45 and55, including terminals, and such that up to 55% of said active acidsites of said agent are neutralized by sodium ions. (iii) adding agrinding substance to said suspension; and (iv) grinding said mixturethus constituted; wherein the size of at least 90% of the particles inthe ground substance is less than two microns, and the size of at least65% of them is less than one micron.
 2. The aqueous suspension of finemineral substances according to claim 1, wherein the Brookfieldviscosity of said suspension as measured without stirring after at leasteight days of storage does not exceed 1,510 mpa·s at 10revolutions/minute.
 3. The aqueous suspension of fine mineral substancesaccording to claim 1, wherein the size of at least 80% of the particlesin the ground substance is less than 1 micron.